1. Field of the Invention
The present invention concerns a printing unit with a doctor blade chamber and a rotatable roller where the roller extends partially into the doctor blade chamber for receiving ink from the doctor blade chamber during operation, wherein a gasket is provided for sealing between the doctor blade chamber and the roller, where the gasket at one side includes at least one sealing surface, the sealing surface having a width substantially corresponding to the width of the doctor blade chamber and a thickness in longitudinal direction of the roller, the thickness delimited by a first edge and a second edge at the sealing surface, wherein the gasket at its opposing side includes an elastomeric edge for sealing abutment against the roller, wherein the elastomeric edge is offset in longitudinal direction of the roller/doctor blade chamber relative to at least one of the edges that delimit the thickness on the sealing surface, where a surface extending from at least one of the edges and to an area at the elastomeric edge/roller delimit the thickness of the sealing surface, wherein the surface is inclining corresponding to it being angled less than 90° in relation to the sealing surface or the longitudinal direction of the roller/doctor blade chamber, is concave and/or is convex. The invention further concerns a gasket for sealing between a doctor blade chamber and a rotatable roller in a printing unit.
2. Description of Related Art
A rotary printing unit for colour printing or for application of lacquer on printed matter typically comprises a semi-open doctor blade chamber with ink which is transferred to a screen roller in that the screen roller while rotating runs with part of its surface submerged in the ink bath in the doctor blade chamber. In order to seal the semi-open chamber towards the ink roller, the doctor blade chamber is connected with a doctor blade bearing against the roller. The ends of the doctor blade chamber are sealed with rubber gaskets which are embedded in the doctor blade chamber with their bottoms and which have a curving top side in contact with the roller. Such rubber gaskets can also be used for dividing the doctor blade chamber into several sections with ink baths.
In EP 401 250 is disclosed a doctor blade device which is represented in FIG. 1. The device includes a chamber bar with a U-shaped doctor blade chamber 3 with bottom 3A and sides 3B and 3C which during operation contain ink for a printing unit with a screen roller (not shown) which is in contact with the ink in the chamber 3. Two doctor blades 1, 2 are clamped to the chamber 3 by rails 5, 6, having the function of sealing against the screen roller, its surface being in contact with the ink in the chamber 3. Ink can be conducted into the chamber 3 via channels 8. Two channels 8 are shown, one for each part chamber in the chamber 3, wherein the part chambers are provided by delimitation by means of a packing 4 inside the chamber and a packing 4 at the end of the chamber. The packing 4 has a concave shape 4A for bearing against the screen roller.
A packing of this type is reproduced in FIG. 2 which is a copy of the U.S. design Pat. No. D 488,503. This packing has a central section with a sharp rail 7 of a hard material for bearing against the screen roller for enhanced sealing. Packings or gaskets with rails for bearing against the screen roller are also described in WO 2009/024151.
In general, long life of the sealing and the tightness between the doctor blade chamber and the roller are important aspects and subject to continuous improvement. For example, it is desirable with improved durability of the packings or gaskets, and at the same time it is desired that the gaskets are more tight in general, even in case of incipient wear. Even though the above described hard rails in the gaskets are an improvement compared with the prior art, these gaskets have the drawback that the colour pigments are wearing considerably on the hard rails. Making these rails even harder may reduce wear but implies a risk of more rapid wear on the screen roller at the line of contact with the rail. It is thus desirable to use a type of gaskets where the service life is high and wherein tightness is improved during normal operation and up to the point where the gasket/packing is worn out.